1. Field of the Invention
The present invention concerns a battery containing in a metallic casing at least one electrode pair comprising a separator, a positive electrode, a separator and a negative electrode, the assembly being impregnated with electrolyte, the electrodes of the same kind being connected in parallel and to respective output terminals.
2. Description of the Prior Art
When it is metallic, the casing of a battery is very often used for one of the poles.
In this case, the skilled person knows that the metal of said casing must be chemically and electrochemically stable relative to the electrolytic solution at the potential imposed on it by the electrode of the battery to which it is connected.
For example, it must not be oxidized when it is at the high potential of the positive electrode.
In the case of batteries with high energy density and a non-aqueous electrolyte the positive electrode materials can be very strongly oxidizing. For example, lithium manganese oxide in the form of spinel LiMn.sub.2 O.sub.4 must be at a potential of at least 4.3 V relative to a lithium electrode to be charged sufficiently. Cobalt oxide and nickel lithium oxide must be at a potential of 4.2 V or 4.1 V, respectively, to be charged.
Under such conditions nickel, nickel-plated steel or stainless steel at such potentials in a non-aqueous electrolyte consisting of solvents such as alkyle carbonates and a lithium salt such as LiPF.sub.6, LiBF.sub.4, LiClO.sub.4, LiCF.sub.3 SO.sub.3 or any other type of lithium salt is oxidized electrochemically.
Ni, Fe, Cu ions pass into solution and have a very negative effect on the stability and the self-discharge of the battery. The casing ultimately perforates because the corrosion is mainly in the form of pin-holes. It is consequently necessary to select stable metals. The only one known at present is aluminum which is also used for the current collectors of the electrodes in this type of battery.
The casing can be connected to the negative electrode. In this case it is also necessary to assure that no unwanted electrochemical reaction occurs at the surface of the metal at the reducing potential in an electrolytic solution. In the case of batteries with a lithium anode or an anode material, such as carbon, with inserted lithium, this potential is very strongly reducing relative to the solvent. The latter reacts and this forms a layer having ionic conduction properties at the interface with the metal.
Depending on the nature of the metal, this layer can be passivating, that is say self-inhibiting vis-a-vis the reaction, to a greater or lesser degree. If the passivation is insufficient the reaction of reduction of the electrolyte by the negative active material (Li or substance with inserted lithium) continues, leading to self-discharge of the element. This phenomenon is accentuated if the temperature is raised.
To avoid these constraints, when the configuration of the battery allows, two output terminals insulated from the casing can be used; the casing is then at a "floating" potential that is not determined by the potential of an active electrode. This configuration can be of benefit if it is required to avoid a large conductive surface at the battery potential, as for example in the series connection of high-capacity batteries.
However, this solution also has a drawback: not being fixed by an active electrode, the potential can take any value within the range of electro-activity of the electrolyte. The latter can be very high in the case of non-aqueous electrolyte batteries (up to 4 volts or 5 volts).
Consequently, even very small quantities of impurities in the electrolyte can cause an undesirable potential at the metal/electrolyte interface at which depassivation reactions can occur in the case of a stainless steel, nickel or aluminum casing, for example, or a casing of any other metal that is stabilized by a passivating oxide layer. This depassivation can then be accompanied by corrosion, in particular pin-holes.
Very strongly reducing materials can act at the surface of the metal to degrade it. This occurs with accidental deposits of lithium through the growth of dendrites from the negative electrode. If the casing is of aluminum an alloy with formula LiAl is formed, degrading the mechanical properties of the aluminum and possibly culminating in perforation of the casing.
Casings made from material proof against such attack have been proposed to solve these problems, like the special steel disclosed in Japanese patent application N.sup.o 241113 of 1993 or in document FR-A-2 260 876. Such materials are costly and their high weight reduces the energy per unit mass.
The aim of the invention is to propose a battery with a metallic casing the positive and negative terminals of which are insulated from the casing and in which there is no risk of corrosion and none of the aforementioned drawbacks.